The base band signa E.sub.B for an FM broadcast wave is obtained by adding a modulated signal obtained by amplitude-modulating a subcarrier (2f.sub.p =38 kHz) by a stereo signal E.sub.S to a monaural signal E.sub.M 0.about.15 kHz, as indicated in FIG. 6, and a carrier is frequency-modulated (FM) by this base band signal E.sub.B. Further a sinusoidal wave of f.sub.p =19 kHz is also superposed as a pilot signal on the base band signal E.sub.B.
Consequently, the right ear side signal being represented by E.sub.R, the left ear side signal by E.sub.L, which are fundamental signals, the following relationships are valid: EQU E.sub.M =1/2(E.sub.R +E.sub.L) (1) EQU E.sub.S =1/2(E.sub.R -E.sub.L) (2) EQU E.sub.B =E.sub.M +E.sub.scos2.omega.pt+pcos.omega.pt ( 3)
where pcos.omega..sub.p t represents the pilot signal. The waveform obtained by removing the pilot signal from the base band signal E.sub.B is indicated in FIG. 5.
FIG. 7 shows the basic construction of a prior art FM stereo receiving device. In the figure, reference numeral 1 is a receiving antenna; 2 is a high frequency amplifier; 3 is a local oscillator; 4 is a frequency converter; 5 is an amplitude limiter; 6 is an FM demodulator; 7 is a pilot separating filter; 8 is a subcarrier generator; 9 is a switch circuit; 10 and 11 are low pass filters (LPF); and 12 and 13 are an E.sub.R and an E.sub.L outputting terminal, respectively.
In the receiver described above, as indicated in FIG. 7, the output of the receiving antenna 1 is high-frequency-amplified by the high frequency amplifier 2 and inputted in the frequency converter 4. In this way a signal representing a frequency difference between the high-frequency-amplified signal and the output of the local oscillator 3 is outputted by the frequency converter 4. This signal is called the IF (intermediate frequency) signal (frequency f.sub.10 .apprxeq.10.7 MHz). Since it is an FM modulated wave, the level thereof can be limited to a certain constant value by the amplitude limiter 5. The output of the amplitude limiter 5 is demodulated by the FM demodulator 6 to obtain the base band signal (E.sub.B in Equation (3)). The pilot signal f.sub.p =19 kHz is taken out from this base band signal by the pilot separating filter 7, which pilot signal is inputted in the subcarrier generator 8 (for which a PLL circuit, etc. are used) to generate the subcarrier cos2.omega..sub.p t. In this way the switch circuit 9 is switched. The other output of the pilot separating filter 7 is a wave, from which the pilot signal f.sub.p =19 kHz (pcos.omega..sub.p t in Equation (3)) is removed. Since this is given by E.sub.M +E.sub.S cos2.omega..sub.p t, this is inputted in the switch circuit 9, which is switched by the output cos2.omega..sub.p t of the subcarrier generator 8. At point A in FIG. 5(a), since cos2.omega..sub.p t=-1, the right ear side signal E.sub.R is obtained. On the other hand at point B in FIG. 5(a), since cos2.omega..sub.p t=-1, the left ear side signal E.sub.L is obtained. Since the output of the switch circuit 9 is a pulse train, E.sub.R and E.sub.L are taken out by means of the low pass filters 10 and 11, which are sent to the output terminals 12 and 13, respectively.
Although FIG. 7 is a scheme of the construction representing the principle, this is utilized in practice exclusively with an extremely simple construction.
However the signal FM-demodulated by the FM demodulator 6 indicated in FIG. 7 generates triangular noise having a noise distribution in the base band, as indicated in FIG. 2, which increases proportionally to the frequency of the base band, peculiar to the FM modulation method.
Consequently, because of the triangular noise increasing at high frequencies, as indicated in FIG. 2, the SN ratio differs considerably for E.sub.M (monaural) below 15 kHz and the stereo signal of 38 kHz.+-.15 kHz. Therefore the SN ratio of the stereo signal was lowered significantly with respect to the SN ratio of the monaural signal.
Because of this lowering in the SN ratio of the stereo signal, there was a problem that the service area of the stereo broadcast was restricted. That is, the prior art receiver indicated in FIG. 7 had a drawback that the SN ratio is worsened when receiving the stereo channel E.sub.S in a weak electric field.